That continuous low roar under a thrash chorus is not one foot working overtime. It is two. Double bass means a second pedal, either a slave pedal linked to your main one by a driveshaft or an entire second bass drum, so the feet alternate R L R L the same way your hands alternate in a single-stroke roll. The four exercises here climb the ladder the rest of this track is built on: one kick per beat, then two, then four, then a groove sitting on top of them.
All four write the kicks on one staff line, so nothing on the page tells you which foot played which note. The sticking does that, and from Exercise 2 on it is strict alternation with your lead foot on the beats. Sort out one piece of hardware before you count in. Once both feet are on bass pedals your left foot has left the hi-hat pedal, so lower the clutch until the top cymbal rests on the bottom one — nothing else is holding the hats shut while your hand plays them.
Climbing from one kick a beat to four
The hand part is identical in all four exercises: straight 8ths on the closed hat, eight strokes a bar, start to finish. It is the control variable, so when the sound changes, the feet changed it. Exercise 1 puts a single kick under every other hat stroke at ♩ = 90, one foot only, and it is the only one here you can play on a single pedal. Exercise 2 holds that tempo and brings the second foot in on the & of every beat, so every kick now has a hat stroke sitting directly on top of it. A flam between hand and foot has nowhere to hide there, which is why the tempo does not move: the second foot is the only new thing in the bar.
Exercise 3 is where the feet break away from the hands. Sixteen kicks a bar against eight hat strokes, and with R L R L starting on beat 1, the R foot keeps landing with the hat on the beats and the &s while the L foot takes every e and every a. The weak foot has just lost its cover. Its notes are the only ones in the bar with no hand stroke over them, so any softness in it reads as a limp in the 16th stream. The written tempo drops to ♩ = 80 here, because hand 8ths against foot 16ths is a coordination problem before it is a speed problem, and even with the drop the foot rate climbs from 180 kicks a minute to 320. Exercise 4 changes nothing underneath and adds the backbeat on 2 and 4 above it, and that combination is the sound the first three exercises have been building toward.
Work them in order, and give Exercise 1 more time than it looks like it deserves. Two minutes of quarters at ♩ = 90 with the leg still loose is the stamina every exercise above it borrows from. On Exercise 3, start well under the written tempo, 60 is a reasonable place to begin, and move the click up only when the 16ths sound like one continuous stream instead of loud-soft-loud-soft. Do not press the beater into the head and hold it there on every stroke. That buried sound is a real metal choice at slower kick rates, but 16 to a bar is too many to hold down; let the beater rebound and use the bounce to start the next one.
Where the second foot gives itself away
Speed is not what fails first. Volume is. Drop your hands, play the feet of Exercise 2 alone for 30 seconds, and record it. If you hear a pulse every other kick, your L is quieter than your R and what you are playing is an 8th-note stream accented on every beat, not an even one. More reps at that tempo will not fix it; reps at a slower tempo where both feet sound identical will. The second failure happens upstairs. When the feet jump to 16ths the hand wants to go with them, rushing the hat or leaning into it, and the graduation bar for Exercise 4 asks for a steady hat while the kicks run underneath. The moment the hat drifts, stop the feet, play four bars of hat alone until it settles, then bring the feet back in.